1 files changed, 77 insertions, 0 deletions
diff --git a/dns_random.hh b/dns_random.hh
new file mode 100644
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--- /dev/null
+++ b/dns_random.hh
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+/*
+ * This file is part of PowerDNS or dnsdist.
+ * Copyright -- PowerDNS.COM B.V. and its contributors
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ *
+ * In addition, for the avoidance of any doubt, permission is granted to
+ * link this program with OpenSSL and to (re)distribute the binaries
+ * produced as the result of such linking.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+#pragma once
+#include <cstdint>
+#include <limits>
+#include <string>
+
+void dns_random_init(const std::string& data = "", bool force_reinit = false);
+uint32_t dns_random(uint32_t n);
+uint16_t dns_random_uint16();
+
+namespace pdns {
+  struct dns_random_engine {
+
+    typedef uint32_t result_type;
+
+    static constexpr result_type min()
+    {
+      return 0;
+    }
+
+    static constexpr result_type max()
+    {
+      return std::numeric_limits<result_type>::max() - 1;
+    }
+
+    result_type operator()()
+    {
+      return dns_random(std::numeric_limits<result_type>::max());
+    }
+  };
+
+  /* minimum value that a PRNG should return for this upper bound to avoid a modulo bias */
+  inline unsigned int random_minimum_acceptable_value(uint32_t upper_bound)
+  {
+    /* Parts of this code come from arc4random_uniform */
+    /* To avoid "modulo bias" for some methods, calculate
+       minimum acceptable value for random number to improve
+       uniformity.
+
+       On applicable rngs, we loop until the rng spews out
+       value larger than min, and then take modulo out of that.
+    */
+    unsigned int min;
+#if (ULONG_MAX > 0xffffffffUL)
+    min = 0x100000000UL % upper_bound;
+#else
+    /* Calculate (2**32 % upper_bound) avoiding 64-bit math */
+    if (upper_bound > 0x80000000)
+      min = 1 + ~upper_bound; /* 2**32 - upper_bound */
+    else {
+      /* (2**32 - (x * 2)) % x == 2**32 % x when x <= 2**31 */
+      min = ((0xffffffff - (upper_bound * 2)) + 1) % upper_bound;
+    }
+#endif
+    return min;
+  }
+}